It depends on the site, but usually it is between $0.001 and $0.02 per ad click. However, these will not be your essential earnings!
The BIG MONEY you earn from your referrals. The more you have, the more you earn. Again, each site gives different rates about the earnings from referrals. Some give 100% of their earnings, some a smaller amount, some prices are fixed (for example $0.001 per referral click). But with 100 referrals you can earn several hundred dollars per month. And remember the more you have, the more you earn!
So maybe you will ask yourself if I have enough referrals, why should I keep clicking on the ads for some cents, when the referrals make me hundreds of dollars? Well, it's simple the sites want you to be active. In some of the sites they will delete your profile after a certain amount of time of inactivity (you should check that in their terms of use), and in neobux for example, you get money from your referrals for the previous day, only if you have clicked on all your ads this day.
